In the field of toners used for electrostatic latent image development during recent years, there has been rapidly advanced development of an electrophotographic apparatus responsible to meet the requirements of the market and a toner usable in the apparatus. For instance, in response to the requirement for high-quality imaging in the market there has; been required a toner with a narrow particle size distribution. Namely, a toner in which particle size becomes uniform and a narrow particle size distribution is achieved, results in markedly enhanced reproducibility of minute dots. However, it is not easy to achieve a narrow particle size distribution in the toner production method of the conventional pulverization processes.
On the other hand, there was proposed an emulsion aggregation process as a production method capable of controlling the shape or the particle size distribution, of toner particles. In such a process, an emulsified dispersion of resin particles is mixed with a colorant particle dispersion and optionally with a wax dispersion, and the respective particles are allowed to aggregate by addition of a coagulant or pH control with stirring and the thus aggregated particles are allowed to fuse by heating to obtain toner particles.
Further, there has been promoted development of a low temperature fixing toner capable of achieving fixing at a relatively low temperature with the object of energy savings. However, it is necessary to lower the melting temperature or melt viscosity of a binder resin to achieve a lowering of the fixing temperature of a toner. However, lowering the glass transition point or the molecular weight of a binder resin to lower the melting temperature or melt viscosity of the binder resin produced problems such as deterioration of heat storage stability or fixing separability.
There was also informed a technique of controlling a toner to a core/shell type structure to achieve both low temperature taxability and heat storage stability (as described in, for example, JP 2005-221933 A). Namely, a shell layer composed of a resin which exhibits a high softening point and enhanced heat resistance is formed on a core particle which is excellent in low temperature fixability, rendering it feasible to achieve both low temperature fixing and heat storage stability. Specifically, toner production by the emulsion aggregation process also has the advantage that shape control can be easily conducted. In recent production print areas, however, increased speed of a copier and a printer, and expansion of corresponding kinds of paper are promoted and it has become difficult to achieve both low temperature fixing and heat storage stability by the foregoing core/shell type toner.
To solve such a problem, there was developed a toner in which a polyester resin was used for a shell layer (as described in, for example, JP 2005-338548 A). A polyester resin has the advantage of low softening point design making it feasible to maintain a high glass transition point, as compared to styrene-acryl resin. A toner which is superior in low temperature fixing and heat storage stability can be obtained by use of a polyester resin for a shell layer.
However, a styrene-acryl resin and a polyester resin are low in affinity and when a styrene-acryl resin was used for a core and a polyester resin is used for a shell layer, it was difficult to form a uniform, thin shell layer, so that it was impossible to achieve sufficient heat storage stability. Further, there was a problem such that fusion of a core and a shell layer is also not feasible, rendering it difficult to control the shape of toner particles, so that it was difficult to prepare toner particles having homogeneous, close and smooth surfaces; cashing resistance was low and when performing continuous-printing, peeling of a shell layer occurs by stirring a toner within a copying machine, resulting in a large variation in electrostatic charge and causing image noises, leading to deteriorated image quality.
To solver the foregoing problems, there was proposed a toner of a core/shell structure, using a urethane-modified polyester resin and/or acryl-modified polyester resin (as described in, for example, JP 2005-173202 A). There was also disclosed a technique to improve low temperature fixing, offset property and humidity dependence of electrostatic charge by using a resin, as a toner resin, in which a radical polymer unit is linked to a polyester resin through a bivalent cross-linking group (as described in, for example, JP 2011-028257 A).